Severe Gram-positive infections are difficult to treat and associated with high mortality in hospital settings. Daptomycin is a broad spectrum antibiotic that has been successfully used to treat persisting Gram-positive infections, including skin and soft tissue infections, right-sided endocarditis, and bacteremia. Daptomycin is also highly effective against drug-resistant strains such as methicillin-resistant S. aureus (MRSA) and vancomycin-resistant enterococci (VRE). Current daptomycin dosing guidelines are derived from limited studies in healthy individuals, where pharmacokinetics are highly predictable. Unfortunately, daptomycin pharmacokinetics are highly variable in the critically ill patients that typically receive the drug. This leads to sub-therapeutic dosing, increased rates of treatment failure, and in recent years, the emergence of daptomycin-resistant strains. Therapeutic monitoring of daptomycin concentrations would thus enable clinicians to adjust doses in order to maintain an effective circulating concentration, ultimately improving outcomes and preventing adverse effects. Unfortunately, there is no FDA-approved test to measure daptomycin concentrations in a clinical setting. Multiple clinical studies have concluded that current daptomycin dosing guidelines for critically ill and septic patients need to be reevaluated and adjusted, and regular measurement of daptomycin levels in these patients are needed to ensure effective treatment. To address this critical need, Affinergy is developing a novel assay format using daptomycin-binding phage to measure daptomycin concentrations in plasma. At the conclusion of this Phase II project, we will file a regulatory submission for FDA clearance of our assay.